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hey jpmassy i am not an expert but i would try 6061 with that filler or try
5051? with 5000series alum. i am sure that someone with more exp. will jump in and we can both learn a new tip! good luck shoprat

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"Alloys possessing from 1% to 2.5% magnesium, such as 3004 and 5052, are very sensitive to cracking when using base alloy filler. Alloys with 3.5% magnesium and more exhibit low sensitivity to weld cracking. The use of a 5% magnesium content filler, such as 5356, to weld 5052 can provide a higher magnesium percentage in the diluted weld metal and reduce sensitivity to weld cracking. The more magnesium an aluminum magnesium alloy contains, the less likely it is to crack; thus, the high magnesium content fillers 5356, 5183, and 5556 are commonly used to weld both wrought and cast aluminum magnesium base alloys."

Comment

I sorta doubt it was just filler that caused your problem. I've welded loads of 5052 with 4043 and not had any problems. It is not your first choice but it does a good enuff job that a crack such as that is not a common occurance.
I think it is more of a lack of fusion judging from your pic. Looks like maybe moving to slow by the looks and it seems crowned a bit much like filler was added too soon as well. I would strive for more of a keyhole effect myself...That bead will weld with fusion alone. Aluminum will bridge across 2 parts easily without penetrating completely thru unless you make it hot enuff before adding filler and moving forward.
What does the back side of the weld look like would be the question. I realize it is a tank tho, so you prolly can't see it. I would cut out the weld 100% and crank the heat more and let it get wetter before I moved forward. HTH

Comment

That may be tearing from the root. Do you have a keyhole in front of your weld puddle while welding?

Also, are your welds cooling too fast? Have you considered using a little preheat? That sheet of aluminum is an awfully good heat sink for sucking the heat away from the weld rapidly.

80% of failures are from 20% of causesNever compromise your principles today in the name of furthering them in the future."All I ever wanted was an honest week's pay for an honest day's work." -Sgt. Bilko"We are generally better persuaded by reasons we discover ourselves than by those given to us by others." -Pascal"Since we cannot know all that there is to be known about anything, we ought to know a little about everything." -Pascal

"Alloys possessing from 1% to 2.5% magnesium, such as 3004 and 5052, are very sensitive to cracking when using base alloy filler. Alloys with 3.5% magnesium and more exhibit low sensitivity to weld cracking. The use of a 5% magnesium content filler, such as 5356, to weld 5052 can provide a higher magnesium percentage in the diluted weld metal and reduce sensitivity to weld cracking. The more magnesium an aluminum magnesium alloy contains, the less likely it is to crack; thus, the high magnesium content fillers 5356, 5183, and 5556 are commonly used to weld both wrought and cast aluminum magnesium base alloys."

Comment

I sanded and then SS wire brushed it. The edges are plasma cut, so that probably didn't help.

"Plasma Arc Cutting, Beveling, and Gouging:

· This process has some limitations and must be carefully controlled. If it is used, it requires the power source to be set (DCEN) along with the use of small orifices to gain high velocity and concentrated heat. Heat affected zones will be crack prone particularly for 2XXX, 6XXX, and 7XXX series alloys and will require 1/8 inch or more of mechanical surface removal before welding. Series 1XXX, 3XXX, and 5XXX alloys are not as crack prone and can generally be welded as cut by this process."

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I was gonna say that too. Unless you are using a shielding gas while plasma cutting, you have to remove most of the HAZ because of the high levels of oxidization that occur. Even when you use shielding gas vs. comp air, the oxidization penetrates more than you would think. Also, I would use a 5356 filler on a 50xx series aluminum. It also looks like you might need to use a little more balance to clean the weld better.

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I was gonna say that too. Unless you are using a shielding gas while plasma cutting, you have to remove most of the HAZ because of the high levels of oxidization that occur. Even when you use shielding gas vs. comp air, the oxidization penetrates more than you would think. Also, I would use a 5356 filler on a 50xx series aluminum. It also looks like you might need to use a little more balance to clean the weld better.

What machine are you using? What tungsten? Balled or pointed? Pulse?

Syncrowave 250dx
I believe I was using pointed 2% thoriated.
No pulse option.

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The little black pepper looking flakes that float on the top of the puddle turn into stuff that looks like that. Also notice that the white "flash" on the outside of the bead is narrow, meaning only a small amount of DCEN in the cycle, balance adjustment might fix it, but like was said before, if it was plasma cut, there is alot of oxide present, and it needs to be removed before welding.